This acid of formula CH.sub.3 --COSH, manufactured by reaction of hydrogen sulphide with acetic anhydride (patent FR 1,417,154), is currently used as an intermediate for synthetizing pharmaceutical products such as captopryl (antihypertensive agent).
It is known that thioacetic acid has a tendency, on hydrolysis, to form various sulphur-containing or oxygen-containing compounds which lower the thioacetic acid assay of the commercial product. This instability of thioacetic acid is due in particular to the formation of hydrogen sulphide and acetic acid according to the reaction: ##STR1## catalysed by acidic species (Tetrahedron, 1965, vol. 21, pp. 835). Diacetyl sulphide (CH.sub.3 CO--S--COCH.sub.3) and diacetyl disulphide (CH.sub.3 CO--S--S--COCH.sub.3) are also formed.
In order to stabilize thioacetic acid, patent JP 1,345,605 has proposed adding a phosphorus-containing inorganic acid or a strong organic acid (pKa&lt;3.3) thereto. Apart from oxalic acid, the other compounds mentioned (phosphorous acid, citric acid, methanesulphonic acid) do not have a sufficient stabilizing action for good conservation of thioacetic acid on storage.